The present invention relates to an organic EL (ElectroLuminescence) element and, more particularly, to an organic EL element in which an organic EL layer is hermetically sealed with a hermetic sealing cap, and a method of manufacturing the same.
EL is a phenomenon in that a certain type of phosphor emits light upon application of an electric field, and an element that utilizes this phenomenon is an EL element. In the structure of an organic EL element, an organic layer made of a light-emitting material using an organic compound is sandwiched between two electrodes. The arrangement of the organic layer includes a multilayered structure of a light-emitting layer made of a fluorescent organic solid such as anthracene and a hole injection layer made of a triphenylamine derivative or the like. Another organic layer is also available which is comprised of a multilayered structure of a light-emitting layer and an electron injection layer made of a perylene derivative, or a multilayered structure of a hole injection layer (hole transport layer), an EL light-emitting layer, and an electron injection layer (electron transport layer). In the organic EL element, a multilayered structure interposed between two electrodes is generally formed on a substrate.
Such an organic EL element utilizes light emitted when electrons injected to the light-emitting layer are recombined with holes. With this organic EL element, high-luminance light emission of several 100 cd/m2 to several ten 1,000 cd/m2 can be obtained with a voltage of about 10 V, and the response speed is also high.
The organic EL element has a disadvantage in that it is not resistant to water. For example, even a very small amount of water can separate the light-emitting layer and electrode layer from each other, or denature the materials constituting the light-emitting layer and the like. In this case, a non-emission portion called a dark sport is formed to degrade the quality of emission. Therefore, when a conventional organic EL element is driven in the atmosphere, its light-emitting characteristics quickly degrade. In order to obtain a practical organic EL element, the element must be sealed so water or the like will not enter the light-emitting layer.
As a structure for sealing the organic EL element, a structure in which a resin or the like is directly applied to the organic EL element, or a hollow structure filled with a gas or liquid is available. FIG. 3 shows the arrangement of an organic EL element to which a conventional sealing structure (filled-type structure) is applied. In the organic EL element shown in FIG. 3, anodes 13, an organic EL layer 14, and cathodes 15 are sequentially formed on a glass substrate 12. For example, the organic EL layer 14 has a multilayered structure of a hole transport layer and an EL light-emitting layer. In a filled-type structure, the organic EL element formed on the glass substrate 12 is covered with a hermetic sealing cap 16 fixed by an adhesive 41, and the interior of the hermetic sealing cap 16 is filled with an inert gas.
As the material of the sealing cap 16, for example, a glass, metal, or plastic is used. As the adhesive 41 for bonding the glass substrate 12 and sealing cap 16 with each other, a resin that sets at room temperature or a resin that sets upon irradiation with ultraviolet rays is used because the organic EL element has a low heat resistance. When the sealing cap 16 is to be fixed on the glass substrate 12 by using such an adhesive, after the adhesive 41 is applied to the bonding surface of the glass substrate 12 or sealing cap 16, operation of fixing the glass substrate 12 and sealing cap 16 is necessary.
When the organic EL element as described above is to be downsized, the area of a region that does not contribute to light emission must be reduced. A region that does not contribute to light emission includes a portion necessary for sealing adhesion, a region between a sealing adhesion portion and the end of a light-emitting region, an electrode (anode) extraction portion, and the like. An electrode with a thickness of about 100 nm, which is necessary for causing the internal organic EL element to emit light, is formed on the surface of the glass substrate 12. The glass substrate 12 itself has a warp or undulation of at least about several xcexcm.
Hence, the warp or undulation described above and a three-dimensional shape (step) formed by the electrode are present on that portion of the glass substrate 12 which is to be bonded to the sealing cap 16. When the glass substrate 12 and sealing cap 16 are brought into direct contact with each other, they do not come into tight contact with each other but leave many gaps between them. For this reason, the adhesive 41 is applied to a certain constant thickness (10 xcexcm to several 100 xcexcm) to absorb the three-dimensional shape, and the glass substrate 12 and sealing cap 16 are bonded to each other to form no gaps between them. Since the adhesive 41 is applied thick in this manner, when the glass substrate 12 and sealing cap 16 are aligned with and pressed against each other, the sandwiched adhesive 41 may be squeezed and extend to likely attach to the organic EL element.
When the adhesive 41 attaches to the organic EL element, a stress is generated to act on the organic EL element regardless of whether the adhesive 41 is set/unset. The organic EL element which is very thin is thus damaged to form a non-emission portion. For this reason, squeezing of the adhesive 41 must be controlled. When the amount of adhesive 41 is reduced, although squeezing does not occur, the three-dimensional shape cannot be sufficiently absorbed by the adhesive 41, and adhesion becomes poor to form gaps, so hermeticity cannot be held.
The adhesive 41 used for sealing the organic EL element must be set at a temperature which is low to such a degree that it does not degrade the organic EL element. Also, a gas that adversely affects the organic EL element during setting should not be generated. In addition, the adhesive 41 must have a low permeability so that hermeticity can be held over a long period of time.
From the above requirements, the adhesive used for sealing the organic EL element must have the following characteristics:
1. A warp or undulation on the bonding surface between the sealing cap and glass substrate, and a step formed by an extraction interconnection can be planarized sufficiently.
2. The setting temperature (adhesive fixing temperature) is lower than a temperature that the organic EL element can stand.
3. No gas toxic to the organic EL element is generated during setting.
4. The set adhesive has a low permeability.
5. The interface of the adhesive has low adhesion properties and water permeation from the interface with the hermetic sealing cap or glass substrate is sufficiently small.
Although an ultraviolet-curing epoxy resin which does not require heating can satisfy the required characteristics of items 1 to 3, and 5, its permeability is not very low, which is disadvantageous in holding the hermeticity. In contrast to this, although a thermosetting epoxy resin has a lower permeability than that of the ultraviolet-curing epoxy resin, the gas generated during setting degrades the organic EL element, and the temperature necessary for setting exceeds the heat resistance of the organic EL element. Therefore, this epoxy resin does not satisfy the items 2 and 3, and cannot be adopted for sealing the organic EL element.
Therefore, conventionally, the ultraviolet-curing resin is used, although its hermeticity holding level is insufficient from the viewpoint of the service life of the organic EL element. Also, the thickness of the adhesive is decreased as much as possible in order to compensate for the low permeability. When the material of the sealing cap is a conductive metal, partly because of the molding precision of the sealing cap, the thickness of the adhesive cannot be made thin when preventing the organic EL element and the connecting electrode from coming into contact with each other. In this case, water penetration from the thick adhesive layer to the interior where the organic EL element is formed cannot be suppressed very much, and the service life of the organic EL element is shortened.
It is an object of the present invention to provide an organic EL element with a longer service life, and a method of manufacturing the same.
In order to achieve the above object, according to the present invention, there is provided an organic EL element comprising a light-transmitting transparent substrate, an organic electroluminescence (EL) layer formed on an element formation region of the substrate, a frame formed on the substrate to surround the element formation region, the frame having a flat upper surface that has absorbed a three-dimensional shape on a surface of the substrate, a sealing cap to be fixed to an upper surface of the frame, and an adhesive layer for adhering the frame and the sealing cap to each other, the adhesive layer being set at a temperature lower than a heat resistant temperature of the organic EL layer.